Without limiting the scope of the invention, its background is described in connection with a frame transfer device for image sensing, in particular a charge coupled device array for image sensing.
In a typical frame transfer device technology, it is difficult to create a device with an electronic shutter because the time of transfer of the captured data into a memory area is longer than the interval available, that is the time interval equal to the horizontal blanking interval. It is desirable to use a frame transfer device in these applications because the frame transfer CCD's (charge coupled devices) are advantageous to the currently used interline devices in many ways, including the simplicity of the construction, higher resolution, and higher sensitivity than corresponding interline transfer devices.
For example, in the standard video application using the NTSC standards, it is has been observed that a prior art frame transfer CCD arrangement having a 244 line image sensing array, a memory for holding the last captured data, and a serial output register for outputting the line data is best utilized if the array is not disturbed during the horizontal line readout time, because disturbing the sensing array tends to produce interfering signals at the output. This means that the only time available to clear the sensing array and therefore provide for an electronic shutter effect is during the horizontal blanking period. This time is limited by the NTSC standard to approximately 10 microseconds. Because the frame transfer devices of the prior art must be cleared by clocking the charge through each line, the time interval is not long enough to accomplish this goal. The clearing must wait until the longer vertical blanking period. This occurs less frequently, every 1/60 second, which makes an electronic shutter effect difficult to design within the standard.
It is therefore desirable to incorporate an electronic shutter capability into a frame transfer CCD device, for use in image sensing applications currently being implemented by use of conventional interline transfer devices. Accordingly, improvements which overcome any or all of the above discussed problems are presently desirable.